1. Field of the Invention
This invention relates to an image sensor and an image sensing method for acquiring a two-dimensional image. The present invention also relates to a semiconductor device having a CCD (charge coupled device).
2. Description of the Related Art
CCD image sensors and CMOS image sensors made from semiconductor are known as image sensors for converting a two-dimensional image formed by light focused by an optical image forming system into an electric signal and outputting the signal and such sensors are known to be widely used. A CCD image sensor generates image signals using a large number of pixels and reads them to the outside by sequentially transferring them by means of a CCD (charge coupled device). A CMOS image sensor sequentially scans pixels by means a CMOS switch and reads image signals to the outside. Most image sensors of the types under consideration are designed to operate at a frame rate of about 30 frames per second for ordinary moving pictures. On the other hand, there may be cases where a phenomenon continues only for a very short time and an ultrahigh image pickup speed of about a million frames per second is required to catch such a phenomenon. However, it takes certain time for a CCD or a CMOS switch to read image signals from many pixels to the outside and hence it is difficult for such an image sensor to operate at such a high speed. Therefore, there are known techniques for realizing such a high-speed image sensing operation by devising a particular arrangement for image sensors.
A method referred to as parallel/partial read out method is known as a method applicable to COD image sensors. With this method, a number of readout paths are provided and the read time necessary for reading image signals is reduced by thinning pixels when a high-speed pixel reading operation should be conducted to realize a high-speed image sensing operation. Image sensors having about 300,000 pixels and adapted to pick up images at a rate of 10,000 frames per second by using this method are commercially available.
Other known image sensing methods include an IS (in-situ storage) method. With this method, a large number of image signal recording elements are formed around the photoelectric conversion section of each pixel. The image signal recording elements temporarily store the image signals obtained by the photoelectric conversion section. Then, the image signal obtained by a single photoelectric conversion section in a short integration time can be stored in one of the image signal recording elements around the photoelectric conversion section. Thus, the image signals, each obtained in a short integration time, can be sequentially stored in the image signal recording elements. Subsequently, when an image sensing operation using a large number of frames ends (after storing image signals in all the image signal recording elements of a pixel), the image signals can be read to the outside. If the image signal reading operation takes a long time, the image signals can be read properly because the image signal recording elements store respective image signals. While the image signal intensity may be low because the integration time is short, it can be made sufficiently high by providing the photoelectric conversion section (photodiode) with a large area. With this method, it is possible to conduct a high-speed image sensing operation of about 1 million frames per second.
It is possible to use a CCD that is adapted to transferring image signals and employ each transfer stage of the CCD as image signal recording element. If such is the case, the CCD is employed as memory (CCD memory). FIG. 9 of the accompanying drawings illustrates the configuration of image sensor 400. An instance of a device including 2×2 pixels is illustrated in FIG. 9 and each of the pixels is provided with a CCD memory 402 consisted of eight transfer stages in addition to a photoelectric conversion section 401. If the device is so arranged that the signal charges generated by the photoelectric conversion section 401 are sequentially transferred in the direction of the arrow shown in FIG. 9, the CCD memory 402 can temporarily store eight signal charges. When signal charges are stored in all the transfer stages, they are sequentially read out by a vertical CCD 403, starting from the first signal charge. Thereafter, the signal charges are downwardly transferred to a horizontal CCD 404 and then to the left side in the horizontal CCD 404. Subsequently, they are converted into an electric signal and output by an output section 405. In the case of FIG. 9, each pixel has eight image signal recording elements.
In the above-described instance, the CCD memory 402 is made to meander and contained in a single pixel. The CCD memory 402 can be manufactured like the vertical CCD 403 and the horizontal CCD 404. However, when the CCD is made to meander, the electrode structure is required to inevitably be complex at the point where the transfer direction is changed.
NPL 1 describes an improved arrangement where a CCD is not required to meander to secure the number of image signal recording elements. FIG. 10 illustrates the configuration of the image sensor 500. With this arrangement, the signal charges obtained by a photoelectric conversion section 501 are sequentially transferred to a horizontal CCD 503 by way of an input gate 502 and in the rightward direction in the horizontal CCD 503. Vertical CODs 504 are connected to the respective transfer stages of the horizontal CCD 503. When signal charges are stored in all the transfer stages, which are five in total, of the horizontal CCD 503, they are transferred to the respective vertical CCDs 504 all at the same time. As this operation is repeated, the horizontal CCD 503 and the vertical CCDs 504 operate as CCD memory. In the instance of FIG. 10, the number of image signal recording elements each pixel has is 30 in total that are formed by the horizontal CCD 503 having five transfer stages and five vertical CCDs 504, each having five transfer stages. The signal charges are read out as they are transferred to the outside of the illustrated region at the bottom side of the vertical CCDs 504 in FIG. 10.
Since the image signal recording elements are constituted only by the horizontal CCD 503 and the vertical CCDs 504, the arrangement is less complex if compared with the first known arrangement.
NPL 2 describes an image sensor realized by using CCD memories that extend in a direction slightly inclined from the vertical direction and connected to vertical CCDs instead of a horizontal CCD and vertical CCDs of NPL 1. FIG. 11 illustrates the configuration of the image sensor 600. The illustrated image sensor 600 has 3×3 pixels. A CCD memory 602 slightly inclined from the vertical direction is connected to a photoelectric conversion section 601. The CCD memory 602 linearly extends in that direction and is connected to a vertical CCD 603 at a position located below the pixel to which the photoelectric conversion section 601 belongs. The signal charges obtained by the photoelectric conversion section 601 are transferred downwardly in the CCD memory 602 and converted into an electric signal and read out by way of the vertical CCD 603 and a horizontal CCD 604. The vertical CCDs. 603 and the horizontal CCD 604 operate like their counterparts in FIG. 9.
Since the CCD memories 602 can be made linearly long with this arrangement, it is possible to provide a larger number of image signal recording elements (a large number of transfer stages). Additionally, this arrangement is structurally simple and can be manufactured with ease because only linear CCDs are employed. This image sensor 600 can successively and continuously pick up 144 images at a speed of one million frames per second with 300 thousands pixels.
An image sensor for high-speed image sensing operations can be obtained by using an IS (in-situ storage) type sensor having the above-described structure.